The present invention relates to a method for manufacturing half-tone image areas required in printing in conformity with original pictures of continuous tone, and more particularly to a method for manufacturing half-tone image areas consisting of many small dots which can be treated by the dot etching technique developed for the half-tone process.
The original pictures of continuous tone have heretofore been converted into the half-tone images by means of the image scanning recorder such as panchromatic or monochromatic scanner. To put it concretely, either a method utilizing a contact screen or a method called the dot generator system has been used, the latter method being an electronic technology for making said small dots. In the former photographic method, a large intensity of exposure light as well as a complicated vacuum apparatus for the vacuum contact are needed so that the apparatuses as a whole becomes large-sized and expensed. The other problem in the photographic method exists in the application of lith-films the exposure and processing of which should be very precisely controlled. There may be a disadvantage also in the known electronic method such as shown in the Japanese Patent Publication No. Sho. 52-33523. Namely, each of the small dots thus produced has a so extremely small fringe width that it is classified as a hard dot which is not suitable for an original film utilized in plate making process, whereas the photographically produced dots have sufficiently wide fringe zones surrounding the central main portions thereof. The lack or scarcity of the fringe is caused by the fact in said electronic method that both the central and circumferential portions of each of the small dots are exposed evenly to the light of the same intensity. The scarcely fringed dots cannot be processed by the dot etching technique required for the color retouching additionally performed depending on the result of color separation tests. In said known dot etching process, the silver layer forming the small dots of half-tone film is etched partially or entirely over the whole image area in order to render the dots smaller (however, in the half-tone negative film, the dots becomes larger in the printed objects). The abovementioned hard dots can be dot-etched only to an unsufficient degree of effect because the deposited silver amounts are not so varied between the central and the circumferential portions in each of said dots. That is to say, it is difficult to render the dots smaller gradually from their circumferential portions, and if the dot etching is forcedly continued the silver amount of the central portion in each dot will be undesirably reduced at the same time so that the half-tone image consisting of such dots becomes unfit for use. For the purpose of solving the above problem, an apparatus has been proposed in the U.S. Pat. No. 4,025,189, the apparatus being constructed such that the exposure intensity of light beam to the circumferential portion in each of said small dots is controlled to be lower than that to the central portion. The small dots produced in this manner are considerably similar to the gradated dots that have been photographically produced. In the other words, the proposed apparatus is suitable for making half-tone images consisting of such dots that have fringes on their circumferential portions, which fringes can be dot-etched, if necessary. In the above apparatus, the sums of (or the differences between) the image signal values originating from the original picture and the signal value from the screen pattern generator are made use of. The above sums (or differences) for the translational light beams positioned at the right and left hand, in close proximity to each other, are compared with one another for each beam to judge the relation therebetween. In case that both the sums (or differences) indicate the same condition, i.e. exposure or non-exposure, the light beam will be controlled to be in full exposing or non-exposing condition. In the other case, namely when they indicate different conditions for the right and left beams, a value c will be calculated by means of the following equation; ##EQU1## where a and b are respectively the sums (or differences) for the right and left beams. According to the level of said value c, the exposure is effected with a variable intermedium intensity. The apparatus, therefore, must comprise a complicated circuitry including an adder (or subtraction circuit), an absolute valve circuit together with a division circuit. The digital processing for obtaining the quotients reeds a considerably longer time so that the apparatus would be considered more or less unsuitable, in future, for utilization in the scanners for plate making process where there is a requirement or need of a higher speed.